It is understood that the agreement between the clubs was to process the payments in instalments over three years.
It is understood that Cardiff received a letter from Nantes on Tuesday, in which the French club threatened to take legal action if the first scheduled payment is not received within the next 10 days.
Cardiff City had agreed to a club record 15m pounds to sign the striker from the French club.
Cardiff will honour the deal but not until they have clarified "all the facts".
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And Cardiff are "surprised" Nantes have made the demand while attempts are being made to recover a body from the plane wreck that was carrying Sala and pilot Ibbotson.
Sala was on board the Piper Malibu that tragically crashed into the English Channel after the striker joined the English Premier League team in a $19 million (£15million) deal.
A Nantes supporter stops by a poster of Argentinian player Emiliano Sala outside La Beaujoire stadium after the footballer went missing in January.
Bordeaux will also receive a percentage of the fee, thought to be 50 per cent, because Sala played for them from 2012-2015 before signing for Nantes.
A body was spotted in underwater images of the wreckage and Britain's Air Accidents Investigation Branch (AAIB) has launched an operation to try and recover it.